This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 09 675.9, filed on Mar. 5, 1999, the entire disclosure of which is incorporated herein by reference.
The invention relates to a composite layer structure for use on structural components subject to a danger of erosion damage, including at least one fiber-reinforced synthetic material layer as a substrate, and a cover layer that includes at least one layer of metallic fibers and/or threads arranged adjacent to the substrate. The invention further relates to a method of manufacturing such a layer structure.
Layer structures or laminates embodied in the manner of fiber-reinforced composites combine the advantages of a low weight as well as a sufficient strength. Such layer structures based on fiber composite materials, however, have a relatively low resistance to erosive wear. For this reason, such fiber composite layer structures must be protected against erosion when they are used in applications in which they are subjected to fluid mechanical impingement, for example when they are used in the fluid flow path of a fluid flow machine, whereby abrasive particles carried in the fluid as well as varying thermal loading of the components would otherwise have a deleterious effect on the fiber composite layer structure. For this reason, various forms of erosion protection for such a layer structure are known in the art.
German Patent 42 08 842 discloses an erosion protection for use on helicopter rotor blades made of fiber-reinforced synthetic materials. Specifically, the erosion protection includes a metal sheet that is glued or adhesively bonded onto the critical areas of the rotor blade that are especially subjected to erosive wear, and the metal sheet is coated with particles of metallic compounds. In this context, it has been discovered that the reliability and durability of the adhesive bonding of the metal sheet onto the underlying fiber composite structure is problematic and inadequate in practice.
German Patent Publication DE 196 27 860 C1 discloses a blade for a fluid flow machine and a method for its manufacture, whereby in at least partial areas, layers of fiber-reinforced synthetic material are protected with a cover layer of metallic fibers or threads, and this cover layer is bonded or connected using the same synthetic resin binder as the layers of fiber-reinforced synthetic material. In this manner, the desired high adhesion of the cover layer onto the underlying layers of fiber-reinforced synthetic material is ensured. It is desirable, however, to further improve the mechanical properties and particularly the abrasive wear and delamination resistance as well as the erosion resistance of such a structure that is to be exposed to high loads.
German Patent Publication 196 42 983 A1 discloses a layered body or laminate with a substrate, as well as a method for manufacturing it. The layered body includes at least one fiber layer and a cover layer adjacent thereto, wherein the cover layer comprises metallic fibers or threads at least on the side thereof adjoining the fiber layer, and wherein the metallic fibers or threads are saturated or impregnated with a binder agent just as the fiber layer. In order to improve the erosion resistance, particles are embedded in the cover layer in the area of the metallic fibers or threads. Such a layered body, however, does not provide an adequate adhesion ability for various functional layers that are to be applied thereon without chemically or physically attacking the resin matrix. Thus, such a layered body must be further improved with regard to its adhesion ability, especially in combination with functional layers to be applied thereon.
German Patent Laying-Open Publication 19 00 477 discloses a composite part comprising a layer of fiber-reinforced synthetic material and a metallic carrier or substrate, on the outer surface of which a layer of metal fibers in the form of mats, fleeces or woven webs is welded or soldered. The metal fibers establish a form-locking connection with the synthetic material and are saturated or impregnated with a binder together with the synthetic material.
In view of the above, it is an object of the present invention to provide a layer structure of the above described general type, which is improved in such a manner so that the outer surface or skin thereof can be used as a functional outer surface and, for example, provides a good adhesive base for further functional layers applied thereon, whereby the underlying resin matrix is protected during the application as well as the removal of the functional layer or layers. Another object of the invention is to improve the adhesion between an outer metal skin and an underlying fiber-reinforced composite substrate by improving the continuous mutual penetration of a binder agent or matrix material throughout the substrate and the layer of metal fibers and/or threads of the cover layer. It is also an object of the invention to provide a method of manufacturing such a layer structure. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as are apparent from the present specification.
The above objects have been achieved in a composite layer structure according to the invention, comprising a substrate and a cover layer arranged on the substrate. The substrate comprises at least one layer of fiber-reinforced synthetic material including reinforcing fibers and a synthetic resin matrix material. The cover layer includes at least one layer of metallic fibers and/or threads adjacent to the layer of fiber-reinforced synthetic material of the substrate. The metallic fibers and/or threads are impregnated with a synthetic resin binder, and particularly the same synthetic resin binder of the fiber-reinforced synthetic material of the substrate. The cover layer further includes an outer skin comprising a metal sheet that forms an outer surface of the structure and that is connected to the underlying layer or layers of metallic fibers and/or threads at least at a partial area.
The layer of metallic fibers and/or threads has a variable porosity that increases in a direction from the metal sheet or skin toward the fiber-reinforced synthetic material of the substrate. Stated differently, the packing density of the metallic fibers and/or threads increases in a direction from the substrate toward the metal sheet or skin of the cover layer. The varying porosity (or alternatively the varying packing density) may be provided by a porosity (or density) gradient in a single layer of the metallic fibers and/or threads, or especially may be achieved by stacking or layering and laminating a plurality of successive layers of metallic fibers and/or threads, whereby the respective porosity of the successive layers varies layer-to-layer.
Throughout this specification, it should be understood that the terms xe2x80x9couter surfacexe2x80x9d or xe2x80x9couter skinxe2x80x9d or the like can also refer to an exposed surface that is inwardly facing or directed, depending on the particular configuration and construction of the component at hand. For example, if the component is a hollow cylindrical pipe, an xe2x80x9couter surfacexe2x80x9d or xe2x80x9couter skinxe2x80x9d can include the radially inwardly facing interior surface of the pipe wall.
Also, in this specification, the xe2x80x9cporosityxe2x80x9d of the layer of metallic fibers and/or threads refers to the proportional void space between the fibers and/or threads, especially before impregnation with the binder or matrix material. It should be understood that in the finished layer structure, the binder or matrix material may substantially or entirely fill the void spaces between the metallic fibers and/or threads, so that the finished structure has few or no remaining vacant xe2x80x9cporesxe2x80x9d, and a contrary implication should not be taken from the term xe2x80x9cporosityxe2x80x9d. The xe2x80x9cpacking densityxe2x80x9d is essentially the inverse of the xe2x80x9cporosityxe2x80x9d and refers to the volume proportion of the metallic fibers and/or threads relative to the void spaces therebetween.
The terms xe2x80x9cfiberxe2x80x9d and xe2x80x9cthreadxe2x80x9d are used herein to refer to elongated configurations of metallic particles. According to ASTM definitions, a xe2x80x9cfiberxe2x80x9d has a length-to-width aspect ratio of at least 100, and a length of at least 0.5 cm. In the present application, however, a particle will still be regarded as a xe2x80x9cfiberxe2x80x9d even if it has a somewhat lower aspect ratio, e.g. an aspect ratio of at least 10, or preferably at least 20, or more preferably at least 30, or even more preferably at least 50. A xe2x80x9cthreadxe2x80x9d has a higher aspect ratio than a xe2x80x9cfiberxe2x80x9d, e.g. an aspect ratio of at least 500. It should also be understood that individual xe2x80x9cfibersxe2x80x9d are preferably not continuous throughout the length, width and thickness of a layer thereof in the inventive structure, while xe2x80x9cthreadsxe2x80x9d may be so continuous.
In comparison to prior art arrangements in which a metal sheet is glued or adhesively bonded onto a fiber-reinforced synthetic material substrate, the inventive layer structure achieves a very high adhesion of the cover layer onto the underlying substrate, through the metallic fibers and/or threads of an interface layer of the cover layer. On the one hand, the metallic fibers and/or threads of the interface layer are directly secured to the metal sheet forming the outer surface or skin of the cover layer, for example by soldering or sintering. On the other hand, the metallic fibers and/or threads are integrally bonded with the underlying fiber-reinforced synthetic substrate layer by the mutual penetration of the synthetic resin matrix material through the interface layer of metallic fibers and/or threads as well as through the substrate layer of fiber-reinforced synthetic material. Furthermore, the metallic fibers and/or threads may be intimately intertwined or intermeshed with the fibers of the underlying fiber-reinforced synthetic of the substrate, for example by simple lay-up and compressing of the successive layers onto each other, or by purposely inter-needling the metallic fibers and/or threads into the underlying fiber-reinforced synthetic material. The mutual, integral penetration of the synthetic resin matrix material is especially enhanced by the porosity gradient or packing density gradient of the at least one layer of metallic fibers and/or threads. Moreover, the layer structure having a metal sheet as an outer skin provides a good id adhesive base for the application of further functional layers or coatings, for example which may be thermally sprayed or galvanically deposited onto the metal skin, while the matrix lying under the metal sheet is protected against chemical and/or physical attack.
In addition to providing a protection against erosion and chemical attack and degradation of the underlying fiber-reinforced composite materials in the final end use application of the layer structure and during application of functional coatings, the metal sheet forming the outer surface or outer skin provides several further advantages as well. For example, the metal sheet forming the outer skin protects the layer structure and particularly its synthetic matrix against the penetration of liquids or gases into the matrix during the cleaning or delacquering of the layer structure using solvents, other chemicals, and/or hot steam. The metal sheet skin also protects the matrix from being directly subjected to fire or heat influences, for example in the event of an accidental fire near the component including the inventive layer structure. Thereby the metal skin helps protect the synthetic matrix against overheating and/or thermal breakdown. Furthermore, the metal sheet skin can be structured, textured, patterned, or otherwise embodied as a functional surface. For example, a so-called xe2x80x9cshark skinxe2x80x9d pattern or texture can be embossed on the metal sheet skin in order to reduce the aerodynamic resistance of corresponding components of an aircraft, for example the skin of the fuselage or of the lifting surfaces.
In one embodiment, the metal sheet and the metallic fibers and/or threads are connected with each other by sintering or soldering, whereby the metal sheet and the metallic fibers and/or threads are securely bonded together to form the cover layer, which in turn can be bonded with good adhesion onto the substrate comprising at least one layer of fiber-reinforced synthetic material, by the mutual penetration of a binder such as a synthetic resin through the metal fibers and/or threads as well as the reinforcing fibers of the synthetic substrate material. Due to the sintering, the metallic fibers and/or threads are welded or melted to each other, i.e. the metallic fibers and/or threads form an interconnected porous mass or network as well as being firmly bonded to the metal sheet. As a result, the metallic fibers and/or threads are not easily separated due to external loads being applied to the layer structure, or even due to cracking of the resin matrix itself. Thus, the inventive layer structure maintains its structural integrity, even when it is subjected to extreme loads, for example as a result of pressure applied to the outer surface formed by the metal sheet (e.g. in the case of a crankshaft), impact of a foreign body such as a stone or the like against the outer surface formed by the metal sheet, handling errors during assembly and installation, or collisions of aircraft incorporating the inventive layer structure with other vehicles (such as a ground maintenance vehicle accidentally colliding into the fuselage of an aircraft). Under such severe loading or impact conditions, the metal sheet skin may be dented or otherwise deformed, and the underlying resin matrix reinforced by the metallic fibers and/or threads may be correspondingly deformed. Nonetheless, the above described secure bonding of the fibers and/or threads with each other to form a network, as well as the secure bonding of the metal sheet skin to the metallic fibers and/or threads, and the secure bonding of the metallic fibers and/or threads to the underlying fiber-reinforced synthetic material layer of the substrate, provide a significant strength and resistance against rupture or fracture of the layer structure.
Preferably, the metal sheet for the skin and the metallic fibers and/or threads can respectively be made of metallic materials having similar or the same melting temperatures. For example, suitable materials include alloys of steel or nickel.
According to a further embodiment of the invention, the cover layer may comprise two metal sheets that are spaced apart from each other, with at least one intermediate layer of metallic fibers and/or threads arranged therebetween. In such an arrangement, the metallic fibers and/or threads serve to bond or connect the two metal sheets with each other. The outer one of the two metal sheets forms the outer surface or skin of the layer structure. Channels, tubes, passages or other conduits for conveying a cooling medium therethrough can be arranged in the space between the two metal sheets. Such an arrangement provides an extremely good cooling of the surface of the layer structure, as well as a good thermal protection for the underlying fiber-reinforced composite material.
In order to achieve an especially good adhesion of the cover layer onto the underlying substrate, the layer or layers of fiber-reinforced synthetic material forming the substrate and the metallic fibers and/or threads of the cover layer are preferably saturated or impregnated and thereby bonded by the same binder material, for example a synthetic resin, which permeates continuously through the voids between the reinforcing fibers of the substrate as well as the voids between the metallic fibers and/or threads of the cover layer. As a result, the various fibers are embedded in the continuous integral resin matrix in the finished structure after the resin has cured. The degree of embedding does not have to be 100% solid. The binder may be a mixed polymer product of synthetic resin or resins, for example, epoxy resin, phenolic resin, and/or polyimides.
The metallic fibers and/or threads of the cover layer can be provided in the form of a non-woven and non-oriented fleece or felt, and/or a braided or knitted fabric or mat, and/or a woven web, fabric or mat. The use of a fleece or felt of metallic fibers and/or threads is particularly advantageous, because the non-oriented mass of the fibers of the fleece allows an elastic deflection or deformation of the fibers in response to an impact of a foreign object against the inventive layer structure, which absorbs and dissipates the impact energy and reduces the risk of a fiber rupture. If a fiber rupture should occur nonetheless, the rupture damage will remain locally limited, since each fiber of a fleece does not extend continuously throughout the entire fleece layer, but rather has a limited length, in contrast to a woven web of threads, in which each thread generally extends continuously through the entire width or length of the woven web. The layer or layers of metallic fibers and/or threads of the cover layer may also include a combination of a fleece layer with a braided or knitted layer and/or a woven layer.
As described above, the porosity of the layer or layers of metallic fibers and/or threads of the cover layer increases in a direction toward the underlying substrate layer of fiber-reinforced synthetic material. Thereby, the penetration and saturation of the binder agent and particularly the resin matrix material into the layer of metallic fibers and/or threads is improved and simplified, so as to improve the overall matrix bonding effect. Preferably, the outermost layer or portion of the metallic fibers and/or threads adjacent to the metal sheet forming the outer skin has a porosity of approximately 95% or less, while the metallic fibers and/or threads adjacent to the underlying fiber-reinforced synthetic material of the substrate have a higher porosity greater than 95%, e.g. of at least 97%. Alternatively, the overall metallic fibers and/or threads of the cover layer have an average porosity of about 95%, with a lower porosity adjacent to the metal sheet and a higher porosity adjacent to the substrate.
The cover layer can be formed on more than one side of the substrate. In other words, the cover layer may even essentially completely enclose and surround the substrate on all sides thereof. Significant for the invention is simply that the cover to layer is provided on at least one side or one surface of the substrate.
The above objects have been further achieved in a method of manufacturing a layer structure according to the invention, wherein the metallic fibers and/or threads are connected to the metal sheet forming the outer skin by means of sintering or soldering, and thereafter the at least one layer of fiber-reinforced synthetic material of the substrate is bonded together with the metallic fibers and/or threads using a binder agent such as a synthetic resin that is impregnated mutually and continuously into the metallic fibers and/or threads as well as the synthetic reinforcing fibers of the substrate. In a particular embodiment of the method, the metallic fibers and/or threads are set simply with a contact pressure against the metal sheet, essentially without any additional compressive pressure, in a two-part mold of coated steel. Then, the arrangement is heated to a temperature approximately 10xc2x0 C. below the respective melting temperature of the metal materials, whereby the metallic fibers and/or threads are sinter-bonded or solder-bonded to the metal sheet.